An EL device, which includes an emissive layer sandwiched between a pair of electrodes consisting of an anode and a cathode, and from which light emission is electrically obtained, has been actively researched not only for application to a display device, but also for application to a variety of light sources, such as a flat-type illumination source, a light source for optical fiber, a backlight for liquid crystal display, or a backlight for a liquid crystal projector.
Particularly, in recent years efforts have been focused on an organic EL device (Organic Light Emitting Diode), in view of its advantages of high light-emitting efficiency, capability of being driven under low-voltage, light weight, and low cost. In the above-described applications as a light source, enhancement in light-emitting efficiency is of primary concern. Accordingly, improvements in the configuration and material for the EL device, a driving method thereof, a manufacturing method thereof, or the like, have been contemplated with an aim of obtaining light-emitting efficiency comparable with that of a fluorescent light.
However, in relation to an in-solid light-emitting device, such an EL device, where light emission is extracted from an emissive layer per se, light emitted at an angle larger than a critical angle, which is determined by a refraction index of the emissive layer and that of a member from which light exits, undergoes total reflection and is confined inside, and thereby lost as waveguide light.
According to a calculation based on Snell's law of refraction, which is the classical theory of refraction, a light extraction efficiency η, which is an efficiency at which generated light is extracted to the outside, is given by the approximate expression: η=½n2, where “n” denotes the refractive index of the emissive layer. When the refractive index of the emissive layer is assumed to be 1.7, η is approximately 17%. That is, not less than 80% of the light is lost as waveguide light; more specifically, lost in the form of light lost in the direction along the device side face.
Meanwhile, in the organic EL device, among excitons generated by means of re-combination of electrons injected from an electrode and holes, only singlet excitons contribute to light emission. The probability of a generated exciton being a singlet exciton is ¼. Accordingly, when merely the above is taken into consideration, the efficiency is not more than 5% and fairly low. However, recent years have seen progress in development of light-emitting materials which are capable of emitting light also from phosphorescence from triplet excitons as a method for improving light-emitting efficiency of an emissive layer per se (Reference 1). Accordingly, potential for radical enhancement in quantum efficiency has been seen.
However, even when the quantum efficiency is improved, light-extraction efficiency reduces the resulting light-emitting efficiency in a ratio multiplied by the quantum efficiency. In other words, improvement in the light-extraction efficiency may bring about a radical increase in the light-emitting efficiency as a synergetic effect.
Examples of methods to extract the waveguide light to the outside include: formation of a region disturbing reflection angle and refraction angle of light (hereinafter referred to as “region disturbing reflection/refraction angle of light”), thereby changing variables of Snell's law and changing a transmission angle of light which is supposed to undergo total reflection; and imparting a light-condensing property to light emission per se. However, formation of such a region which allows all the waveguide light to be guided to the outside is not easy. Therefore, a number of proposals have been put forth for extracting as much waveguide light as possible.
For instance, as methods for improving light-extraction efficiency, there have been proposed a method for enhancing light-extraction efficiency by means of imparting a light-condensing property to a substrate per se (Reference 2); a method of configuring the emissive layer from discotic liquid crystal, to thus improve frontal directivity of emitted light per se (Reference 3); and a method of forming a stereo structure, an inclined surface, a diffraction grating, or the like in the device (References 4, 5 and 6). The above proposals, however, entail problems such that configuration is complicated, and light-emitting efficiency of the emissive layer per se is low.
Examples of a relatively simple method include formation of a light diffusion layer so as to change the refraction angle of light, to thus reduce the amount of light which undergoes total reflection. For instance, there have been made a number of proposals such that a diffusion plate wherein particles are dispersed in a transparent substrate, and the particles have a structure of gradient-refractive index, which is a structure in which the refractive index on the inner side differs from that of the surface side (see Reference 7); a diffusion member in which a single-particle layer is arranged on a translucent substrate (see Reference 8); and a method of dispersing scattering particles in the same material as that of the emissive layer (see Reference 9).
The above proposals have been made by focusing on characteristics of scattering particles, a refractive index difference from a dispersion matrix, a dispersing form of particles, a location for forming a scattering layer, and the like.
In addition, there has been proposed a method for improving a diffusing function of the light-scattering film for use with a liquid crystal display apparatus (see Reference 10); more specifically, a method of dispersing inorganic particles in a resin so as to increase the refractive index difference, to thus improve a diffusing function. However, the proposals do not include such a concept of extracting lost light, which is supposed to be confined inside the EL device and lost as waveguide light, thereby improving light-emitting efficiency.
Meanwhile, as is the case with the EL device, with regard to a light-emitting device which is configured such that an organic thin film or an inorganic thin film where an emissive layer is disposed between a pair of electrodes, a transparent electrode is employed for an electrode on the light extraction surface. For the transparent electrode, indium tin oxide (ITO), which is obtained by doping tin oxide with indium oxide, is widely employed, in view of its excellent transparence and electric conductivity.
The refractive index of ITO varies, depending on its composition, deposition method, or crystal structure, and is in the range of approximately 1.9 to 2.0. That is, ITO is a material having considerably-high refractive-index.
On the other hand, for a transparent substrate for use in an organic EL device, glass is generally used in view of its excellent transparency, strength, gas barrier property, chemical resistance, heat resistance and so on. The refractive index of typical soda lime glass, for example, is about 1.52.
A light emitting material used for the emissive layer of the organic EL device and an organic layer such as an electron transfer material, a hole transfer material generally have a refractive index of about 1.65 to 1.75, which is relatively high as compared with a typical organic material because of the π-conjugated bond system including many benzene rings in the molecular structure.
In such an organic EL device, light generated in the emissive layer is directed to all spaces. When the above-described relationship of the refractive index exists, total reflection occurs not only at the interface between the glass substrate and the air layer but also at the interface between the ITO layer and the glass substrate.
Specifically, in FIG. 7, provided that the refractive index of the emissive layer is 1.7, that of the ITO layer is 1.9, that of the glass substrate is 1.52, and that of the air layer is 1, total reflection does not occur when the light is transmitted from the emissive layer to the ITO layer, because the refractive index of the ITO layer is higher than that of the emissive layer, so that all the light enters the ITO layer except the light reflected at the surface. However, since the refractive index of the emissive layer is higher than that of the glass layer, there exists a critical angle.
For this reason, light having a transmission angle larger than the critical angle undergoes total reflection at the interface between the ITO and the glass substrate, and is confined inside the device. Further, incident light entering the glass substrate undergoes total reflection at the interface between the glass and the air, to thereby be confined inside the device. When ratios with regard to the above are calculated in consideration of the solid angles, light which is allowed to exit to the outside totals approximately 20%, light which is reflected at the interface between the glass and the air totals approximately 35%, and light which is reflected at the interface between the ITO and the grass totals approximately 45%. Hereinafter, the interface between A and B is referred to as “A/B interface”.
Accordingly, when an organic EL device is configured as described above, even when a light diffusion layer is formed on a glass substrate, light allowed to be extracted therefrom is merely that reflected at the glass/air interface. That is, the configuration exerts no effects on light reflected at the ITO/glass interface. Additionally, as described hitherto, calculation based on classical theory shows that approximately 45% of the emitted light is lost at the ITO/glass interface.
Examples of conceivable methods for solving the above problem include a method of: employing a glass having a high refractive index equal to or greater than that of the emissive layer as a glass substrate, and forming a light diffusion layer on the surface; a method of forming a light diffusion layer made from a high-refractive-index material between the ITO and the glass substrate; and a method of inserting a high-refractive-index layer which is sufficiently thicker than a wavelength of the light, and forming the aforementioned light diffusion layer on the surface of the high-refractive-index layer.
However, the high-refractive-index glass has a problem in that its cost is generally high. Furthermore, manufacturing a light diffusion layer or a microlens structure made from a highly refractive material requires a resin material of excellent workability. However, a general resin material, even that of a highly refractive material, has a refractive index not greater than 1.65. A special resin whose refractive index is approximately 1.7 is available; however, such a resin is considerably expensive.
In addition, a thin, high-refractive-index layer having a thickness of not more than 1 μm can be manufactured with relatively ease by means of a thin film deposition method such as a vacuum deposition method, a sputtering method, or a sol-gel method. However, forming a highly refractive layer which is sufficiently thicker than the wavelength of light, as described above, is significantly difficult because of problems, such as deposition speed of the film, or generation of cracks caused by internal stress. Therefore, exception has arisen for a material which is inexpensive and easily applicable.
An organic EL device relies upon the principle that holes injected from an anode and electrons injected from a cathode are recombined by application of an electric field to form excitons and thereby cause a fluorescent (or phosphorescent) substance to emit light. In order to achieve an improved quantum efficiency, therefore, it is necessary for such recombination to take place efficiently. For that purpose, it is usual to make an element in the form of a laminated structure. Examples of the laminated structure include: a two-layer configuration having a hole transfer layer and an electron transfer emissive layer; and a three-layer configuration having a hole transfer layer, a emissive layer and an electron transfer layer. There have also been proposed many laminated elements having a double heterojunction structure for improved efficiency. Hereinafter, a laminated configuration having layer A, layer B and layer C is referred to as “configuration of layer A/layer B/layer C”.
In a laminated structure, recombination occurs almost exclusively in a certain region. In an organic EL device of the two-layer type as mentioned above, for example, it occurs intensively in a region 6 which is separated from by about 10 nm to the electron transfer emissive layer from an interface between a hole transfer layer 4 and an electron transfer emissive layer 5 which are sandwiched between a reflective electrode 3 and a transparent electrode 2 on a support substrate 1, as shown in FIG. 10 (as reported by Takuya Ogawa, et al.: IEICE TRANS ELECTRON, Vol. E85-C, No. 6, page 1239, 2002).
The light generated in the light emitting region 6 is radiated in all directions. As a result, there occurs a difference in light path between light radiated through the transparent electrode 2 toward its light-emitting surface and light radiated toward and reflected by the reflective electrode 3 and thereby radiated toward the light-emitting surface, as shown in FIG. 11.
Referring to FIG. 11, the electron transfer light-emitting layer of an organic EL device usually has a thickness in the range of several dozen to a hundred and several dozen nanometers, that is in the order of the wavelength of visible light. Accordingly, beams of light going out finally interfere with one another and are strengthened or weakened by one another, depending on the distance d between the light emitting region and the reflective electrode.
While only light radiated in a frontal direction is shown in FIG. 11, there actually exists light radiated obliquely, and interference occurs in a different way depending on the distance d, the wavelength λ of emitted light and the angle of radiation. As a result, it is possible that beams of light radiated in a frontal direction are strengthened by one another, while beams radiated at a wide angle are weakened, or vice versa. In other words, the luminance of emitted light depends on the visual angle. It is, of course, true that as the distance d increases, the intensity of light varies more remarkably with the angle. Therefore, the film thickness is usually so selected that the distance d may be equal to about one-fourth of the wavelength of emitted light to ensure that beams of light in a frontal direction be strengthened by one another.
If the distance d is smaller than, for example, about 50 nm, the reflective electrode which is usually made of a metal absorbs light markedly and thereby brings about a reduction in luminous intensity and an adverse effect on intensity distribution. In other words, an organic EL device has a distribution of radiated light varying markedly with the distance d between its light-emitting region and reflective electrode and a corresponding large variation in the waveguide light component mentioned above.
Moreover, the emission spectrum of an organic electroluminescence device has broad characteristics over a relatively wide range of wavelengths. Therefore, the range of wavelengths in which beams are strengthened by one another varies with the distance d, and thereby causes light to have a varying peak wavelength. The emission spectrum varies with the visual angle, too, depending on the distance d.
In order to solve those problems, it has been proposed that the film thickness be so selected as to suppress any difference occurring to the color of light with the visual angle (see Reference 11). There is, however, no statement concerning waveguide light. Furthermore, the film thickness proposed for suppressing the visual angle dependence of the color of light definitely differs from the range according to this invention as will be stated later.
For the reasons stated above, it is impossible to estimate correctly the output efficiency of a laminated organic EL device by a classical calculation based on the understanding that about 80% of emitted light is confined within the device as waveguide light. In other words, the waveguide light component depends markedly on the structural features of the element. For example, M. H. Lu, et al. report a detailed study made of any variation in waveguide light due to the structural features of the element by using a quantum mechanical method of calculation taking a microcavity effect into aocount (J. Appl. Phys., Vol. 91, No. 2, page 595, 2002).
Further, as described above, there have not yet been put forth proposals which focus on the waveguide light of the EL device; particularly, there have not yet been proposed EL devices in which waveguide light undergoes total reflection at the interface between the transparent electrode and the glass substrate, which extract such light efficiently, and which exhibit improved light-emitting efficiency. Moreover, high refractive index materials applicable for such an application, particularly resin materials exhibiting excellent workability, have not been found in large number.
[Reference 1] JP2001-313178A
[Reference 2] JP63-314795A
[Reference 3] JP10-321371A
[Reference 4] JP11-214162A
[Reference 5] JP11-214163A
[Reference 6] JP11-283751A
[Reference 7] JP6-347617A
[Reference 8] JP2001-356207A
[Reference 9] JP6-151061A
[Reference 10] JP2003-156604A
[Reference 11] JP5-3081A
[Reference 12] EP281381B
In view of above circumstances, an object of the present invention is to provide an EL device which has excellent light-emitting efficiency by virtue of being capable of efficiently extracting lost light which is supposed to be confined inside the EL device as waveguide light. Other object of the present invention is to provide a highly efficient planar light source and a display employing such an EL device as described above.